The invention relates generally to machines for automatically interconnecting terminals with wire and more specifically to a method and apparatus for interconnecting pairs of terminals on large scale circuit boards with a pretwisted pair of insulated wires.
Automatic apparatus for interconnecting selected pairs of terminals disposed in a two dimensional array on a panel are well known in the art and are exemplified by the machines disclosed in U.S. Pat. No. 3,103,735 to J. R. Bos et al. and U.S. Pat. No. 3,185,183 to F. W. Loy. In these machines, a length of single conductor insulated wire is routed between two terminal pins and secured thereto by tightly wrapping uninsulated ends of the wire about the pins in a helical stack. Such machines generally include a pair of substantially identical wrap tools which are mounted on independent carriages for translation along X and Y coordinate axes such that the wrap tools may be moved into alignment with the terminal pins to which connection is to be made. The utilization of only two wiring carriages, of course, limits machine capability to straight-line, point-to-point wiring. Due to the large number of closely spaced terminals which may reside on a circuit board and the wiring density which may accumulate in certain regions of the board, it has been found desirable to route interconnecting wires in patterns other than a straight line in order to avoid and minimize high wiring density regions. Therefore, additional similarly translating dressing finger mechanisms have been incorporated to provide additional moveable points about which an interconnecting wire may be formed in order to achieve a desired wiring pattern. The number of independently controllable dressing finger mechanisms is directly related to the complexity of wiring patterns which the machine may achieve. Typically two or four such mechanisms will be utilized, providing the machine with the capability of achieving complex U and Z shaped patterns.
In spite of the sophistication, speed, and reliability which such machines exhibit, they typically have been limited to the routing and interconnection of a single conductor between two terminal pins. For many applications, however, it is desirable to utilize twisted pair conductors between terminal pins. Twisted pair wire interconnections are desirable for several reasons, the most important of which is the reduction of cross talk. Assuming the wires of a twisted pair constitute the signal and ground return path of a circuit, their electromagnetic fields will be oppositely directed and thus tend to cancel one another. Thus the resultant electromagnetic field of a twisted pair is substantially less than that of either of the individual, isolated conductors. Therefore, a twisted pair wire tends to induce less interference and cross talk in adjacent wires. Similarly, the twisted pair wire is, itself, less subject to induced interference from adjacent wires since its magnetic field is less likely to be disturbed by those of adjacent conductors.
Assuming the wire is routed and terminated in accordance with conventional single conductor wire machine practice, secondary advantages accrue. First of all, a machine will complete approximately twice the number of connections in a given time inasmuch as two wires, rather than one, will be routed and terminated during every wire installation cycle. Secondly, visual inspection is simplified inasmuch as two wires and four connections can generally be verified in slightly more time than it now takes to verify placement of one wire and two connections. Devices for interconnecting pairs of terminals with a twisted pair wire are part of the prior art, however, they suffer from one or more drawbacks which render them less than ideal for their intended purpose. For example, it has been suggested that two untwisted wires be utilized by a substantially conventional machine, which incorporates means for twisting the two wires into a twisted pair prior to termination. Those skilled in the art of wire twisting will acknowledge and appreciate the difficulty of achieving a proper bi-helical wrap of two conductors, especially when short lengths of wire are being utilized to create the twisted pair. Even a slight inequality of length will generally result in the longer wire being wrapped around, not with, the shorter wire. It has been found that unless two wires are twisted one about the other in an accurate, bi-helical fashion, the electromagnetic field generated by passage of electricity therethrough will neither be uniform nor minimal. Thus, while the prior art discloses an apparatus for interconnecting terminals with a twisted pair wire, it does not achieve the major advantage of this wiring scheme, i.e., minimum crosstalk.
Ideally, a machine capable of routing and terminating twisted pair conductors will include components for identifying each conductor of the twisted pair and connecting it to the proper one of a pair of terminals at each end such that the proper signal sense or circuit route is automatically achieved. Prior art devices either disclose identification schemes such as visual examination and responsive manual direction by an operator or wire identification means and associated components unsuitable for use in a wire connecting machine.
As noted above, one general difficulty of a discrete wire-terminal interconnecting scheme may be the buildup of wires, i.e., wire density, in certain areas of the board. Not only is such wiring buildup undesirable from an aesthetic standpoint but it renders both installation and board inspection difficult. Buildup of twisted pair wire is an even more severe problem inasmuch as the random orientation of the wrapped pair will tend to inhibit nesting of the wires and increase the overall cross-sectional area of a plurality of twisted pairs. Prior art twisted pair wiring machines do not have appropriate and sufficient mechanisms capable of achieving complex wiring patterns and minimizing wire buildup.